Lubricating greases are homogeneous products of semi-liquid to solid consistency. Essentially, they consist of a dispersion of a thickener in a liquid lubricant or base oil. In general, the thickener is a significant determinant of the properties of the greases.
ASTM D 288 defines lubricating greases as, “a solid to semifluid product of dispersion of a thickening agent in a liquid lubricant. Other ingredients imparting special properties may be included.” See ASTM “Standards on Petroleum Products,” p. 156, 1950
A grease is a lubricant which has been thickened in order that it remain in contact with the moving surfaces and not leak out under gravity or centrifugal action, or be squeezed out under pressure. Thus a major practical problem is the provision or a structure which will stand up to shear and temperature stresses to which it may be subjected during use. See Vold, Marjorie J., and Vold Robert D., J, Inst. Petroleum Tech., 38, 155-163 (1952).
Polyurea compounds are among the thickeners used in making greases. Often the polyurea compounds are prepared directly in the base oil by the reaction of amines with isocyanates.
Polyalphaolefins (“PAOs”) of different viscosity grades are known to be useful in synthetic and semi-synthetic industrial oil and grease formulations. See, for instance, Chapters 22 and 23 in Rudnick et al., “Synthetic Lubricants and High-Performance Functional Fluids”, 2nd Ed. Marcel Dekker, Inc., N.Y. (1999). Compared to the conventional mineral oil-based products, these PAO-based products have excellent viscometrics, high and low temperature performance and energy efficiency under routine conditions and ordinary replacement schedules.
The viscosity-temperature relationship of lubricating oil in a grease is one of the critical criteria, which must be considered when selecting a lubricant for a particular grease application. Viscosity index (VI) is an empirical, unit less number which indicates the rate of change in the viscosity of an oil within a given temperature range. Fluids exhibiting a relatively large change in viscosity with temperature are said to have a low viscosity index. A low VI oil, for example, will thin out at elevated temperatures faster than a high VI oil. Usually, the high VI oil is more desirable because it has higher viscosity at higher temperature, which translates into better or thicker lubrication films and better protection of the contacting machine elements. In another aspect, as the oil operating temperature decreases, the viscosity of a high VI oil will not increase as much as the viscosity of a low VI oil. This is advantageous because the excessively high viscosity of the low VI oil will decrease the efficiency of the operating machine. Thus a high VI oil has performance advantages in both high and low temperature operation. VI is determined according to ASTM method D-2270-93 [1998]. VI is related to kinematic viscosities measured at 40° C. and 100° C. using ASTM Method D 445-01.
Greases are required to ensure the following: good adhesion, low oil separation, low starting torques, compatibility with synthetic materials, and noise dampening (c.f., Rudnick et al., supra). Most importantly, grease requires good frictional properties. There is a need for a grease meeting all these requirements including improved or lower frictional properties. This invention satisfies that need.